1. Field of the Invention
The present invention relates in general to wireless communications systems and, in particular, to system and method for accurately estimating the earliest arrival of CDMA radio signals, either in the forward or reverse links.
2. Description of Related Art and General Background
Efforts are underway to augment wireless communications systems by adding the capability to locate the position of a particular mobile station (MS). The Federal Communications Commission (FCC) has promulgated a regulation directed to this capability (Docket No. 94-102, third report and order adopted Sep. 15, 1999, released Oct. 6, 1999). This regulation requires wireless carriers adopting hand-held position location solutions to locate the position of a mobile station making an emergency 911 call to within 50 meters for 67% of calls (and to within 150 meters for 95% of calls) by October 2001.
In satisfying this requirement, one approach to determining the position of a MS may be to use the available information at the base stations (BSs) and MSs of a wireless communication system, operating under Code Division Multiple Access (CDMA) schemes. CDMA is a digital radio-frequency (RF) channelization technique that is defined in the Telecommunications Industry Association/Electronics Industries Association Interim Standard-95 (TIA/EIA IS-95), entitled xe2x80x9cMOBILE STATION-BASE STATION COMPATIBILITY STANDARD FOR DUAL-MODE WIDEBAND SPREAD SPECTRUM CELLULAR SYSTEMxe2x80x9d, published in July 1993 and herein incorporated by reference. Wireless communication systems employing this technology assign a unique code to each different communication signal and apply pseudonoise (PN) modulation to spread these communication signals across a common wideband spread spectrum bandwidth. As long as the receiving apparatus in a CDMA system has the correct code, it can successfully detect and select its signal of interest from the other signals concurrently transmitted over the same bandwidth.
FIG. 1 (Prior Art) illustrates a simplified block diagram of CDMA wireless communication system 100. System 100 allows MS 110, typically comprising mobile terminal equipment (TE2 device 102) and a wireless communication device (MT2 device 104) to communicate with an Interworking Function (IWF) 108. The IWF 108 serves as a gateway between the wireless network and other networks, such as the Public Switched Telephone Network (PSTN) and wireline packet data networks providing Internet- or Intranet-based access. MS 110 communicates with BS 106, which is associated with a geographic cell or sector, via the wireless interface Um on the reverse link transmission path. BS 106 is configured to process the communication signals from MS 110. BS 106 may also include, or be associated with, position processing capabilities (e.g., Position Determination Entity (PDE) server mechanisms).
On the forward link transmission path, BS 106 communicates with MS 110 via the wireless interface Um. During forward link transmissions, each BS 106 is capable of transmitting information-bearing signals as well as control signals, such as pilot signals. Pilot signals have a plurality of uses, one of them is to identify the BS 106 best suited to accommodate reverse link transmissions. As such, pilot signals are instrumental in determining which BS 106 to xe2x80x9chand-offxe2x80x9d the reverse link transmission to in order to seamlessly maintain communications as the MS 110 travels across different cells or sectors of cells. Pilot signals also provide a time and coherent phase reference to enable MS 110 to obtain initial system synchronization and facilitate coherent demodulation on the forward link. All pilot signals are subjected to the same PN spreading code but with a different code phase offsets to enable MS 110 to distinguish between different pilot signals coming from different sectors or base stations. Each BS 106 may transmit up to 6 different pilot signals with 6 different PN offsets. Use of the same pilot signal code allows MS 110 to find system timing synchronization by conducting a search through all pilot signal code phases of the same code.
As is well known, signal transmissions traveling across air interface Um may be subject to multipath propagation. As such, MS 110 may first receive a direct (i.e., line-of-sight (LOS)) signal corresponding to the forward link signal transmitted by BS 106, followed by time-delayed and attenuated versions of the same signal due to multipath. There may be situations where the first LOS signal is not received and only the multipath components are present. MS 110 may determine the time of arrival (TOA) and energy of all received pilot signals to identify the earliest useable received pilot signal.
To determine the TOA of the received pilot signals, MS 110 may count and store the number of chips (or fractions thereof) of PN code sequences (i.e., PN chips) that lapse from a reference while the signals were received. MS 110 may then identify the earliest received pilot signal by detecting which pilot signal was received after the smallest number of lapsed PN chips. The reference (or zero arrival time) may in general be an arbitrary mark: because of this, isolated TOA measurements cannot be used directly in position determination algorithms. There is the need of at least two TOA measurements corresponding to pilots coming from different geographical points to overcome this arbitrary error. For instance, by subtracting said two measurements, we get a measurement proportional to the difference between the radial distances of the mobile to the two origins: the common error induced by the ambiguity in the zero timing falls out in the subtraction.
To compensate for the effects of multipath propagation, CDMA systems, such as system 100, employ rake receivers, which process and combine the direct and multipath versions of the forward link pilot signal to generate a better received signal. FIG. 2 (Prior Art) depicts a high-level functional block diagram of a MS 110 receiver 200, including a rake receiver demodulator 225 for coherently demodulating the forward link signals received by MS 110. As indicated in FIG. 2, the radio-frequency/digital converter modulo 205 downconverts and digitizes the received signal from the antenna/producing digital samples. The digital samples are supplied to a rake receiver demodulator 225, which includes a searcher 215.
Searcher 215 is configured to search for signals by sweeping across the samples that are likely to contain multipath signal peaks in steps of one or half-PN chip increments. Searcher 215 then assigns finger correlators 210A-C to the stronger multipath signals. Each finger correlator 210A-C locks onto their assigned multipath signal, coherently demodulates the signal, and continues to track the signal until the signal fades away or the finger correlator 210A-C is reassigned by searcher 215. The demodulated outputs of finger correlators 210A-C are then combined by combiner 220 to form a stronger received signal.
Given the ability to detect the TOA of forward link signals, CDMA systems may, at least in theory, exploit these capabilities to extract MS 110 location information. As noted above, MS 110 is capable of determining the TOA of the received multipath components.
As noted above, the promulgated FCC regulation requires the location of a MS to within 50 meters for 67% of calls. A limitation of current CDMA systems is their inability to estimate TOAs with the necessary resolution to comply with the location requirements. For example, counting lapsed PN sequences to within a tolerance of a PN chip to determine the earliest received pilot signal, is of no consequence in establishing a communications link with the closest BS. However, given the fact that a PN chip corresponds to approximately 800 ns., which translates into a radial distance of 240 meters, such a tolerance clearly fails to comply with the location requirements.
Furthermore, since the LOS signal may not be the strongest signal arriving at the receiver, isolating that first arriving signal will not be a trivial task. Note that using a multipath delayed signal for ranging information will have an inherent error due to the extra delay.
Another limitation of current CDMA systems is the effect of time offset jittering on finger correlators of rake receivers. As noted above, the searcher in a MS rake receiver detects the strongest forward link receive signals and assigns a finger correlator to track and coherently demodulate one of the detected signals. However, due to the resolution on the hardware, finger correlators may experience jitter as they attempt to track their assigned signal. The resolution of finger correlators are typically xe2x85x9 of a PN chip, which translates to jittering jumps of approximately 24 meters. Cumulatively, such effects may compromise the accuracy of the ranging information.
Accordingly, what is needed is a system and method capable of accurately estimating the earliest arrival of CDMA forward and reverse link signals.
The present invention addresses the need identified above by providing a novel system and method capable of accurately estimating the earliest arrival of forward and reverse link CDMA signals.
Although the description will be done for the forward link case where the receiver is the mobile station and the transmitters are the base stations, the method and apparatus of the present invention apply the same in the reverse link case where the base station acts as receiver and the mobile station is the transmitter.
System and methods consistent with the principles of the present invention as embodied and broadly described herein include a base station, or group of base stations, that transmit a plurality of pilot signals and a mobile station configured to receive a plurality of signals corresponding to one of the transmitted pilot signals. The mobile station includes a receiver containing a searcher correlating mechanism and at least one finger correlating mechanism. For each different pilot signal, the mobile station receiver detects the arrival times and energy levels of the multipath signals corresponding to said pilot and constructs a searcher histogram and a finger histogram representing an arrival time distribution of samples. The mobile station receiver processes the samples contained within searcher histogram and finger histogram to generate an estimate of the TOA for the first received multipath component of each pilot. At that point, the mobile station can choose to report all the results (one per pilot) to another entity (base station , PDE , . . . ),or if it has the knowledge of which PN pilot sequences are transmitted from which base stations, further process the measurements, reporting only one measurement per base station, corresponding to the smallest TOA of the pilots belonging to that base station.